Military aircraft (e.g., spacecraft) and other vehicles often use vapor compression (reverse-Rankine) cycle refrigeration systems, or other thermal management systems, to move heat from one location to another. The thermal management systems rely on working fluids such as ammonia, R12, R22, propane, R134a, and R152a, or other fluorocarbons and hydrofluorocarbons (HFCs) to move the heat. However, use of these types of working fluids may sometimes result in low thermal conductivity, low heat of vaporization, and low critical heat flux (CHF). Additionally, some of these working fluids are expensive, are weakly flammable, are difficult to biodegrade, contribute to greenhouse gases, and/or deplete ozone. Additionally, commercial and residential buildings also often use reverse-Ranking refrigeration system. Improved efficiency air conditioning systems are needed for these commercial and residential buildings. The refrigerant typically used in these applications is R-134A. This working fluid has nearly the lowest heat of vaporization of known working fluids. Still, it has become widely used because of its useful vapor pressure and its chemically inert, nonflammable characteristics. Unfortunately, fluorinated refrigerants, like R-134A, have high global warming potentials (often referred to as GWP, which is a relative measure of how much heat a greenhouse gas traps in the atmosphere, and which depends on absorption of infrared radiation by certain species, the spectral location of its absorbing wavelengths, and the atmospheric lifetime of the species), which has motivated a search for alternative refrigerants that have higher performance, are non-flammable, and possess low GWP.
Finding alternative working fluids has also become imperative based on several recent government and commercial initiatives, as well as potential future government and commercial initiatives (e.g., phasing out HFC's).
Water has previously been considered as a working fluid for thermal management systems. Water has over ten times higher heat transport capability than R-134A. Water has a significant drawback that has limited its use until present; its characteristic of expansion upon freezing creates a concern about damage and/or rupture of the thermal devices when exposed to sub-freezing temperatures. However, its high heat transport capability and non-toxicity make it attractive. Water is relatively inexpensive, is non-toxic, non-flammable, has a low ozone depletion potential (often referred to as ODP), has a low GWP, and offers high heat of vaporization, high CHF, and superior thermodynamic performance as compared to most common working fluids. However, water has a low density at low temperature, requires a high compression ratio, and results in high compressor outlet temperatures. Additionally, and as noted above, there is a significant volume expansion that occurs with water upon freezing, which is exacerbated by a relatively high freezing point. Such expansion results in the potential damage and/or rupture of heat transport components. Thus, it would be beneficial if a working fluid could be used that offers the various advantages of water without the problems associated with water, particularly in light of the low temperatures at which many military aircraft and spacecraft operate.